1. Field of the Invention
This invention relates to a touch panel and a coordinates detecting method using a touch panel.
2. Description of the Related Art
A touch panel is an input device which enables a user to input data directly to a display device. The touch panel is arranged on a front face of the display device and used to input data. The touch panel allows the user to input data directly based on the visual information on the display device, and it has been widely used for various applications.
A resistive-film touch panel is well known in the related art. In the resistive-film touch panel, an upper electrode substrate on which a transparent conductive film is formed and a lower electrode substrate on which a transparent conductive film is formed are disposed so that these transparent conductive films are opposed to each other. When an external force is exerted at a point of the upper electrode substrate of the touch panel, the transparent conductive films are brought in contact with each other at the point, and a position of the point where the force is exerted can be detected by the touch panel.
Roughly speaking, resistive-film touch panels according to the related art may be classified into four-wire type and five-wire type. In a four-wire type touch panel, a pair of X-axis electrodes is provided on one of an upper electrode substrate and a lower electrode substrate, and a pair of Y-axis electrodes is provided on the other of the two electrode substrates. On the other hand, in a five-wire type touch panel, both a pair of X-axis electrodes and a pair of Y-axis electrodes are provided on a lower electrode substrate, and an upper electrode substrate functions as a probe for detecting voltage. For example, Japanese Laid-Open Patent Publication No. 2004-272722 and Japanese Laid-Open Patent Publication. No. 2008-293129 disclose respective resistive-film touch panels of five-wire type according to the related art.
Specifically, a description will be given of a five-wire type touch panel according to the related art with reference to FIGS. 1 and 2. FIG. 1 is a schematic diagram illustrating the composition of the five-wire type touch panel according to the related art. FIG. 2 is a cross-sectional view of the five-wire type touch panel according to the related art.
As illustrated in FIGS. 1 and 2, the five-wire type touch panel 200 includes a film 210 (which is an upper electrode substrate) on which a transparent conductive film 230 is formed, and a glass plate 220 (which is a lower electrode substrate) on which a transparent conductive film 240 is formed. The film 210 and the glass plate 220 are disposed on opposite sides of a spacer 250 so that the transparent conductive film 230 and the transparent conductive film 240 are opposed to each other and separated by the spacer 250. This five-wire type touch panel 200 and a host computer (not illustrated) are electrically connected to each other by a cable 260.
In the five-wire type touch panel 200 according to the related art, electrodes 241, 242, 243 and 244 are disposed in edge portions of the four sides of the transparent conductive film 240, as illustrated in FIG. 3A, and an X-direction voltage and a Y-direction voltage are alternately supplied by using the electrodes 241, 242, 243 and 244. When the transparent conductive film 230 and the transparent conductive film 240 are brought in contact with each other at a contact point A, an electric potential Va of the contact point A is detected through the transparent conductive film 230 as illustrated in FIG. 3B, and the coordinates of the contact point A in the X-axis direction and the Y-direction can be detected.
In a case of the five-wire type touch panel mentioned above, detecting the position of a single contact point is possible. However, when the transparent conductive film 230 and the transparent conductive film 240 are brought in contact with each other at plural contact points simultaneously, it is difficult to detect the positions of the plural contact points accurately. The reason of the difficulty is described as follows.
As illustrated in FIG. 4A, by using the electrodes 241, 242, 243 and 244 disposed in the edge portions of the four sides of the transparent conductive film 240, an X-direction voltage and a Y-direction voltage are alternately supplied. If the transparent conductive film 230 and the transparent conductive film 240 are brought in contact with each other at two contact points A and B, the coordinates of an intermediate point between the point A and the point B will be detected.
The above-described method using the touch panel according to the related art is directed to detecting an electric potential at a contact point between the transparent conductive film 230 and the transparent conductive film 240 in order to detect the coordinates of the contact point. As illustrated in FIG. 4B, even when the transparent conductive films 230 and 240 are brought in contact with each other at the two contact points A and B simultaneously, only an electrical potential Vc at the intermediate point between the point A and the point B is detected through the transparent conductive films 230 and 240. Hence, the above-described method determines from the detected potential Vc inaccurately that the transparent conductive films 230 and 240 are in contact only at the intermediate point.